Magnetic material



3 Sheets-Sheet 1 FIG. 2

INVFNTO R F. I? C/OFF/ ATTORNEY March 8, 1938. P. P. CIOFFI MAGNETICMATERIAL Filed Aug. 19, 1932 .ilLml HYDROGEN FIG. 1

, I I ll Match 8, 1938. p, c 0 2,110,569

MAGNETIC MATER IAL Filed 1932 3 Sheets-Sheet 2 1.103000 MAX FIG. /0

70,000 F/G. J s0,000

I l l I I l I 800 .900 I000 [/00 I200 [J00 I400 [5'00 1,000 INVENTOR HBy 01 xg/l/mjd).

A T TORNEY March 8. 1938.

P. P. CIOFFI MAGNETI C MATER IAL Filed Aug. 19, 1932 FIG. 5

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FIG. 8

FIG. 9

5 Sheets-Sheet 5 FIG. 6

INVENTOR F. R CIOFF/ A TTURNEV Patented Mar. 8, 1938 TelephoneLaboratories,

MAGNETIC MATERIAL Paul P. Ciofli, Brooklyn, N. Y., assignor to BellIncorporated, New

York, N. Y.-, a corporation of New York Application August 19,

13 Claims.

tric signaling systems and apparatus A general object of the inventionis to improve the magnetic properties of magnetic materials.

Another object is to produce magnetic materials having high initial andmaximum magnetic permeabilities, low coercive force, low hysteresisloss, and high -resistivity.

A further object of the invention is to produce iron having higherinitial and maximum permeability and lower coercive force-and hysteresisloss, than iron hitherto produced,

A particular object is to facilitate the production of magnetic ironhaving such improved properties when in final form ready for use.

In applicant's former application Serial No. I 325,883, filed December13, 1928 there are ,dis

closed methods of securing desirable properties in magnetic materials byheat treating them in The present application is a continuation in partof the former hydrogenous atmospheres.

application.

Ordinary Armco iron heat treated by suit-able ordinary methods has-aninitial permeability of about 250, a maximum permeability of from 6,000to 8,000, a coercive force of about 1.5 gauss for a maximum induction of12,000 gauss, and a hysteresis loss of about 5,000 ergs per cubiccentimeter per cycle for a maximum induction of 12,000 gauss.

Important improvements have been made hitherto in the magneticproperties of ordinary iron and iron alloyed with other magneticmaterials. For instance, maximum permeabilities of 19,000 and 42,000,coercive forces of 0.29 and 0.15 gau'ss for maximum inductions of 15,000gauss, and hysteresis losses of 916 and 1,025 ergs per cubic centimeterper cycle for maximum inductions of 15,000 gauss have heretofore beenstated to have been obtained by others with pure iron and iron-siliconalloys. respectively, when prepared in a vacuum furnace and annealed invacuum or under non-oxidizing conditions at temperatures of about 1100C. These improvements were achieved by reducing the impurities,

1932, Serial No. 629,507

particularly the carbon, to a minimum. U. S. Patent No. 1,358,810,November 16, 1920 to T, D. Yensen describes annealing an iron-siliconalloy containing 4% silicon at 1100 C. under slightly oxidizingconditions, resulting in a maximum 5 permeability of over 40,000 and ina hysteresis loss of less than 250 ergs per cubic centimeter per cyclefor a maximum induction of 10,000. In U. S. Patent 1,110,010, September8, 1914, W. E. Ruder discloses heating silicon steel in hy- 10 drogen attemperatures of from 1,000 to 1,325 C., thereby reducing the hysteresisloss to 0.0060 watt per pound of steel per cycle (about 1,000 ergs percubic centimeter per cycle) at a magnetic density of B=10,000 gauss. InU. S. Patent 1,156,496, October 12, 1915, the same author disclosesshort-time annealing of silicon steel sheets in hydrogen, nitrogen orother gas serving to protect the material from oxidation, attemperatures from 1300' to 1500 C., for periods 20 of 10 seconds to 5minutes, so that the sheets are heated to a temperature of at least 1000C., for the purpose of improving the magnetic properties.

The disclosures mentioned are concerned eithe with iron melted in vacuum(in especially designed furnaces) or with alloys of iron and silicon;their chief object is to produce improved magnetic materials to besubjected to relatively large magnetizing forces, such as areencountered in power systems and apparatus; consequently, they are notconcerned with initial permeabilities and permeabilities at very weakmagnetizing forces.

In contra-distinction to this, the objects of the present invention arenot accomplished by the treatments mentioned and the materials producedthereby, since one of the most useful fields of application of thematerials treated by the methods of this invention are electro-magneticsystems and apparatus which are to be subjected to low magnetizingforces, as in signaling systems, for instance. Also the initialpermeability and permeabilities at low magnetizing forces of materialstreated in accordance with the present invention are greater andhysteresis loss and coercive forces are lower than those produced by thetreatments mentioned. At the same time the maximum permeability isgreatly increased with a corresponding diminution in hysteresis loss andcoercive force at high inductions.

The publications made by previous workers indicate that they have notsucceeded in carrying their researches to the point of achieving thetechnical advances made possible by the methods of the presentinvention. As against a maximum value of initial permeability of 1,700for ordinary iron, in accordance with the best example claimed in a veryspecial case in the prior art the methods herein described permit valuesas high as 10,400 and maximum permeabilities as high as 280,000 to beobtained.

Generally stated:-

These results are accomplished by heating the magnetic materials in anatmosphere of hydrogen of ordinary purity under pressures ranging from afraction of one atmosphere to over twelve atmospheres at temperaturesapproximating the melting point. In the case of iron, or generally, anymagnetic material having a phase transformation point, the treatmentsubsequent to the exposure to the high temperature may be in accordancewith either one of the following variations:-

(1) The material is cooled slowly to room tem perature;

(2) The first treatment is followed by cooling to 930 C., either fast orslowly, then cooling slowly to 880 C. and then at any cooling rate toroom temperature;

(3) The material is cooled either fast or slow ly to 880 C., maintainedat 880 C. for a period of time, and cooled to room temperature at anydesired rate.

(4) In the case of iron and certain other mag-'- netic materials havinga phase transformation point the material may be cooled to any desiredtemperature below 880 C., i. e., room temperature, and then mechanicallyworked, if desired, then heated to and maintained just above the phasetransformation point for a period of time and cooled at any desiredrate.

(5) Alloys of iron having no phase transformation point may generally becooled from the original treatment at any desired rate.

An important feature of this invention is the attainment of high initialpermeability in magnetic materials in final form and shape for use, as,for example, in iron or other materials used for continuous loading ofconductors while the iron is in situ upon the conductor. Values of 2,000and more may be readily obtained in the case of iron upon a conductor.

Unless otherwise specified, the specimens discussed herein were treatedin wire form 0.040 inch in diameter and all hydrogen gas pressures aregiven in millimeters of mercury absolute pressure.

In the case of iron the desired properties may be produced by firstheating the iron in hydrogen at a temperature between about 1100 C. andthe melting point after which the iron may be further dealt with inseveral ways which will be discussed hereinafter.

After being heated as above for a suflicient length of time, usuallyone-half hour or more, the iron is cooled down to and through thealphagamma transformation point (approximately at 900 C.), and thenreannealed at a somewhat lower temperature, about 880 0., either in ahydrogenous atmosphere or not and cooled at any suitable rate. The lowertemperature anneal is given only when strains are introduced duringcooling from the first temperature of heat treatment.

Prior to reheating, the iron may be rolled,

- Maxi- Iemp. r H dr lmtial Specimen of first 0 ogen permcamum am ananneal pressure bimy 922%?- mm. min. 45 10, 400 39, 000 30 min. 760 8,000 35, 000 8% hrs. 700 9, 49, 000 12 hrs. 760 6, 700 48. 000 13 hrs.760 1, 800 19, B00 41 hrs. 1, 520 1, 900 18, 200

An alternative method is to interrupt the cooling at about 880 C. andmaintain this temperature for a time before cooling. In this case noreheating is necessary.

Another method is to heat the iron in a hydrogenous atmosphere as beforeand cool slowly, particularly between about 920 C. and'890 C.

Tests were. made to determine how small amounts of impurities such assulphur, iron chloride, iron oxide, aluminum oxide, silicon, magnesium,manganese, etc. affect the magnetic ,jproperties of iron.

Examples of specific cases are as follows:

cooled in hydrogen and subsequently annealed at 880 C. in hydrogen for30 minutes.

In 'one aspect the invention involves iron in a substantially purecondition. In another aspect, however, the invention involves certainalloys of iron, which are improved in their magnetic and /or electricalproperties, such as initial and maximum permeabilities, hysteresis loss,coercive force, electrical resistance (resistivity) to a greater orlesser extent, by similar treatment; hence the invention involvestreatment of iron-silicon alloys, ironmolybdenum alloys, iron-nickelalloys, iron-cobalt alloys, iron-molybdenum-nickel ailoys and certainothers.

However, it has been found that an increase in the initial and maximumpermeabilities and in the resistivity may be produced in alloys of ironwhich have no alpha-gamma transformation point with only a single hightemperature heat treatment followed by slow or rapid cooling, afterheating in a hydrogenous atmosphere.

' varying results.

: abilities were 1500 and 13,600, respectively. This Examples ofspecific cases are as follows:

' V] Y Temi in mi- Composition pera- Time erohnis ture um per cubic 79%parts Ni"... C. mm. l6 parts 1400 30min 100 32,500 134,000 22.00 ipartsMo 50% Ni, 50% re. 1400 30mm 100 3,200 44,000 35.00 50% Ni, 50% Fe.-.1400 5hlS 700 10,000 48,000 35.00 81% 101. 0% Fe... 1400 30min 700 3,00003,000 18.3 12;3 1 1400 30min 700 1,500 83,000 15.0- 027 re ,s%M0 140030min 700 2,020 11,500 30.00 00 0 W 1400 80mm 100 0.000 30.500 19.00 00%e,4% s1 1410 30min 700 4,000 10.000 52.00 45 v I 15 hrs. '100 1,3000,000 77.00 7% Mo... 1% Mn...

The nickel-iron-cobalt compositions were cooled slowly from 1380 C. to500 C. in three hours, and from 500 C. to 400 C. in two hours.

It is within'the scope of the invention to melt the iron in hydrogen orother hydrogenous atmospheres. The iron thus melted may be cooled andcold worked as desired and then reannealed at 880 C. to restore theimproved magnetic properties.

In a particular experiment, iron was melted in hydrogen at a pressure ofone atmosphere, then cooled slowly through the freezing point, and thetemperature maintained for a period of about one hour just under thefreezing point with the hydrogen pressure unchanged; the iron was thenrapidly cooled to room temperature and cold drawn to wire.

Hard dra'wn wire thus made from hydrogenmelted iron may be treated invarious ways with For instance, it may be heat treated at 1500 C. for 30minutes followed by a reannealing treatment of the cold worked materialat about 1400 C. This combined treatment resulted in values. for initialand maximum permeabilities of 3000 and 29,000, respectively. Then theytime of treatment at 1400" was'increased to 3 hours, resulting in valuesfor initial and maximum permeabilities of 5000 and 25,000, respectively.With a single treatment at 880 C. for six hours, the initial and maximumpermeindicates that there is a decided advantage gained if thehydrogenization of iron is commenced in the melt, because it is thenpossible to obtain im-: proved magnetic characteristics in the coldworked metal by reannealing at considerably lower temperatures.

In another experiment the iron was melted in hydrogen at atmosphericpressure. The pressure was then reduced to between 0.5 and 1 mm. ofmercury and the iron was allowed to solidify and cool to roomtemperature. The ingot was cold worked from thickness to M plate.Toroidal rings cut from this plate were annealed at 880 C. for 6 hoursand were found to have an initial permeability of 2500 and a maximumpermeability of 52,000.

In a further experiment the pressure was not changed but was maintainedat one atmosphere and after solidification and cooling to roomtemperature the ingot was cold rolled to 4" plate. The toroidalspecimens out from this plate were annealed in hydrogen at 880 C. for 6hours. Their initial permeability was 5000 and their maximumpermeability 56,000.

In another experiment Armco iron was melted and allowed to solidify inan ingot mold in a hydrogenous atmosphere of atmospheric pressure.

The ingot was then heated in air at 850- C. and

hot rolled to a plate of A; inch thickness. Toroidal specimens were thenout from the plate and heated at 880 C. in hydrogen for 4 hours.

' They were found to have initial permeabilities of 5400 and maximumpermeabilities of 40,000.

When melting in hydrogen it is desirable, in order to obtain soundingots, to cool the metal slowly through the solidifying temperature,thus permitting it to give up its absorbed hydrogen. Alternately, byevacuating the furnace atmosphere. prior to solidifying the absorbedhydrogen is liberated and a sound ingot may be obtained by solidifyingthe metal more quickly.

Another method of producing sound ingots from hydrogen melts is tosubstitute for the hydrogen, after the hydrogen treatment, a gas whichis insoluble in the molten metal and has little or no chemical aflinitytoward it, such as helium, for instance. Thus the partial pressure ofhydrogen is reduced to zero and the hydrogen absorbed in the metal isliberated, permitting the metal to be solidified quickly into a soundingot., After cold rolling an ingot made in this manner to inch plate,toroidal specimens cut from this plate were annealed in hydrogen at 880C. and were found to have an initial permeability of 4000 and a maximumpermeability of 45,000.

In the operation of the helium substitution method for producing soundingots, it is not necessary that the partial pressure of hydrogen bereduced to zero; it is only necessary to reduce the partial hydrogenpressure to the point where there is no liberation of absorbed hydrogenon solidifying. This may be accomplished either by partially evacuatingthe furnace or by using a mixture of hydrogen and helium. Thus soundingots have been produced by using a gaseous mixture of hydrogen andhelium in the proportion of 6.6% hydrogen and 93.4% helium,corresponding to partial pressures respectively of 50 mm. and 710 mm. ofmercury.

In one practical embodiment of this aspect of the invention, themagnetic material is melted in a crucible or furnace having the form ofan inverted L in a hydrogeneous atmosphere. The upper branch of theinverted L-shaped crucible is formed to constitute the mold into whichthe material is to be cast. After the melting is completed. thefurnaceis connected to a receptacle containing helium under pressure.The helium gas enters the furnace and sweeps out the hydrogenousatmosphere. The stream of helium gas may also be caused to bubblethrough the molten metal bath, for instance by introducing it into thebath by means of heat resistant tubing. The crucible is then tilted andthe purified metal cast into the mold without coming in contact with anyatmosphere but helium.

It has been found that when iron is meltedin hydrogen at atmosphericpressure and then cold worked, reheating for from two to five hours atabout 880 C. in hydrogen produces an initial permeability of 5000 and amaximum permeability of 58,000. Higher temperatures of heat treatmentproduce further improvements in magnetic characteristics.

More specific methods of treating particular magnetic materials and datarelating to the results attained will now be described with reference tothe accompanying drawings. Legends on these drawings and numericalvalues given in the text relative to magnetizing forces, coercive forcesand flux densities are in c. g. s. units and hysteresis losses are inergs per cubic centimeter per cycle.

Fig. 1 of the drawings schematically illustrates an apparatus employedfor quickly and conveniently heat treating magnetic materials inaccordance with this invention.

Figs. 2 to 9 and 11 depict permeability vs. magnetizing force curves forvarious materials produced by diiferent treatments.

Fig. 10 shows the relation which exists between the temperature of thefirst heat treatment and the maximum permeability.

Fig. 12 shows one example of application of material produced by themethods of this invention, namely its use as loading materialinductively associated with a signaling conductor.

In Fig. 1 glass cylinder 20 has a conductive wire 2| sealed in its lowerend, whereas its upper end is closed by a removable air-tight stopper 23through which passes brass tube 24. Tube 24 which is open at 28 for thepurpose of admitting gas into cylinder 20 is connected both to a supplyof hydrogen (not shown) and to a pressure indicating device 25. Rubbersleeve 26 is provided for the purpose of insuring a tight connectionbetween tube 24 and hydrogen supply tube 21. The upper end of thespecimen 29 of the material to be heat treated is attached by means ofclip 35 to brass tube 24, while its lower end dips into mercury 22. At apoint located some distance above the level of the mercury the glasstube is connected by means of tube 30 to a vacuum pump (not shown), forevacuating the air from cylinder 20. A source of alterating current 3|is provided for heating the sample; it is connected through resistance32 to the primary of transformer 33, the secondary of which is connectedto brass tube 24 and conductive wire 2| as shown. The intensity of theheating current may be measured by ammeter 34 and varied by resistance32. Direct current for heating has also been used with identicalresults.

After the sample has been placed into the heat treating tube asdescribed, heating current is passed through it by means of adjustingresist ance 32, and hydrogen or its equivalent is simultaneouslyadmitted through tube 21 and the pressure regulated by means of thevacuum pump acting upon tube 30 and the rate of inflow of gas throughtube 21 so that the material is heated in the .hydr'ogenous atmosphereto a desired temperature.-

In one specific case, a specimen of Armco iron having the form of a wireabout 1 millimeter in diameter and about 60 centimeters long wasinserted into the heat treating vessel and hydrogen admitted andregulated at a pressure of 45 millimeters of mercury. Simultaneously thespecimen was heated to a temperature of about 1500 C. for about 35minutes; then the heating current was cut off and the specimen allowedto cool in its receptacle at an average rate of about 750 C. per minuteuntil it reached room temperature. Magnetic measurements indicated thatit had an initial permeability of 600 'and a maximum permeability of7,500. The specimen was then reinserted into the heating vessel andagain heated in hydrogen at the same pressure to 880 C. kept at thistemperature for 30 minutes, and then the heating current so adjustedthat it cooled at an average rate of 180 C. per minute. Curve A of Fig.2 illustrates the variation of permeability with small varyingmagnetizing forces for this sample. The initial permeability was 10,400,the maximum permeability 39,000, the coercive force 0.10 gauss for amaximum induction of 13,000 gauss and the hysteresis loss 470 ergs percubic centimeter per cycle. Curve B of the-gsame figure graphicallydepicts the initial and maximum permeabilities of the same materialtreated identically except that the hydrogen was at atmosphericpressure. In this case the initial permeability was 6500- and themaximum permeability was 41,000.

An alternative method which yielded similar good results consisted inheating and cooling the sample as mentioned in the preceding paragraph,but interrupting the first cooling operation at about 880 C.,maintaining this temperature for about 30 minutes, and then cooling theroom temperature at an average rate of about 180 C. per minute.

The curve of Fig. 3 shows the permeability vairiation with increasingmagnetizing forces of an alloy containing 4% molybdenum, 79.5% nickeland the remainder iron when given a single treatment consisting inheating to about 1400 C. for 30 minutes in hydrogen at atmosphericpressure and cooling to room temperature at an average rate of about 50C. per minute. It exhibited an initial permeability of 33,000 and amaximum permeability of 134,000 whereas the same material heat treatedby pot annealing at 1100 C. followed by very slow cooling has an initialpermeability of'about 20,000 and a maximum permeability of about 75,000.

The curve of Fig. 4 pertains to an ironsilicon alloy containingapproximately 4% silicon, heated to about 1410 C. (the melting point isat about 1450 C.) for about minutes in hydrogen at a pressure of 45millimeters and cooled at an average rate of about 1000 C. per minute toroom temperature. This treatment produced an initial permeability of4000, a maximum permeability of about 15,500, a. hysteresis loss of only1.97 ergs per cubic centimeter per cycle for a maximum induction ofabout 550 gauss and aresistivity of 52 microhms per cubic centimeter.

The curve of Fig. 5 illustrates the variation in permeability withincreasing magnetizing force of a sample of Armco iron containing 0.3%manganese, heated to 1500 C. in hydrogen at atmospheric pressure andcooled to room temperature at an average rate of 750 C. It was thenreannealed at 880 C. for one hour and cooled at an average rate of 10 C.a minute. It had an initial permeability of 2,600 and a maximumpermeability of 29,000.

The curve of Fig. 6 shows the variation in permeability with varyingmagnetizing forces of a sample of Armco iron containing 0.5% iron oxide,F6203. This sample was heat treated at 1500 C. in hydrogen atatmospheric pressure for 30 minutes, then cooled to room temperature atan average rate of 750 C. per minute, reheated in hydrogen at an averagerate of C. It exhibited an initial permeability of 5,200 and a maximumpermeability of 36,000.

Curves A and B of Fig. 7 are permeability vs. magnetizing force curvesof samples of Armco iron containing respectively 1% and 3.75%molybdenum. Both specimens were heated in hydrogen at atmosphericpressure at about 1400 C. for 30 minutes and cooled at an average rateof 800 of 14.3 and specimen B a resistivity of 19 microhms percubiccentimeter.

The curve of Fig. 8 depicts the variation in permeability with varyingmagnetizing forces for a sample of Armco ironheated for 30 minutes toabout 1500 C. in an unexplosive mixture of about 30% hydrogen and about70% nitrogen at atmospheric pressure, cooled to room temperature at arate of 800 C. per minute, reheated to 880 C. for about 60 minutes andcooled at a rate of about 10 C. per minute to room temperature. Thematerial had an initial permeability of 900 and a maximum permeabilityof 22,300. Mixtures of 50% and 10% hydrogen, remainder nitrogen, havebeen tried with practically the same result. When using gaseous hydrogenand nitrogen together as'thus described,

it is known that some of the gases combine to form ammonia.

The curve of Fig. 9 illustrates the permeability of iron melted inhydrogen at atmospheric pressure, cooled in hydrogen at an average rateof about 150 C. per minute to room temperature, cold worked to wire ofabout 1 millimeter diameter, annealed in hydrogen at atmosphericpressure at 1500 C. for 30 minutes, cooled at a rate of about 750 C. perminute to room temperature, reheated in hydrogen at the same pressure at880 C. and cooled at an average rate of about 180 C. per minute. Theiron had an initial permeability of 4700 and a maximum permeability of27,000.

The curve of Fig. 10 shows the relation which exists between thetemperature of the first heat treatment and the maximum permeability. Itmay be noted that with Armco iron the hydrogen treatment does notproduce any appreciable improvement in the maximum permeability untilthe temperature is raised to about 1350f C.

Fig. 11 illustrates the variation of permeability with magnetizingforces of Armco iron melted in hydrogen, cooled slowly to room.temperature, cold rolled, reheated at 880 C. for about 18 hours inhydrogen, and cooled to room temperature in about one hour.

The heat treatments in accordance with this invention may be selected toproduce desired variations in the magnetic properties in any particularinstance. The treatments may also be modified to correspond to thecharacteristic behavior of the materials to be treated or with thenecessities of commercial production. For instance, it has been foundthat for best results pure iron requires a second heat treatment atabout 880 C. if it has been cooled rapidly from a high temperature. If,however, the iron is alloyed with other materials, such as with 4%molybdenum or 4% silicon, for instance, then the alloy does not requirea second heat treatment for the best results. Without a second heattreatment an initial permeability as high as 4000 for silicon steel and6000 for 4% molybdenum iron has been obtained.

' Furthermore, as a general rule the second heat treatment at about 880.C. need not necessarily -be done in hydrogen for producing improvedinitial and maximum permeabilities but may be carried out in vacuum, orin a non-oxidizing or neutral atmosphere, such as nitrogen, forinstance. However, the best results have so far been obtained whenhydrogen is used.

Thus, a specimen of Armco iron was heated at 1500 C. in hydrogen atatmospheric pressure for- 30 minutes and cooled at an average rate ofiron for 30 minutes at about 1500" C. in hydrogen at atmosphericpressure, cooling it to room temperature at the average rate of about900 C. per minute, reheating it for about 30 minutes in hydrogen atatmospheric pressure at about 880 C.', and finally cooling at an averagerate of about 300 C. to room temperature. This iron had a hysteresisloss of only 1.15 ergs per cubic centimeter per cycle at a maximuminduction of 450 gauss (inductions of this magnitude are-frequently usedin signaling apparatus), together with an initial permeability of 4000and a maximum permeability of 41,600.

Noteworthy physical properties of, materials treated in accordance withthe methods of this invention are remarkable softness and low tensilestrength. By way of example, the Rockwell B hardness of representativesamples of iron was found to vary between +10 and 10 compared to theRockwell B hardness of 70 of pure vacuum-annealed iron. (For definitionsand principles of Rockwell hardness test see S. A. E. Handbook,September 1927, page 270, et seq.).

The properties produced in any given material by the methods of theinvention are functions of at least four variables, to wit: 1) thetemperature at which the material is heated; (2) the duration of theheating; (3) the pressure of the hydrogenous atmosphere in which it isheated; (4) the rate of cooling to the phase transformation point (inthe case of materials having such a transformation point) and the gaspressure in which it is cooled. For instance, varying only factors (1)and (2), the pressure being that of the atmosphere, it was found inthecase-of iron that while heating for 30 minutes at a temperature of about1500 C. produced an initial permeability of 6000, an initialpermeability of about 7000 was obtained by heating at 1400 C. for sevenhours, an initial permeability of 2000 by heating at 1300 C. for 14hours, and an initial permeability of only 1000 by heating at 1200 C.for 20 hours; or, varying factor (3) heating the iron at 1500 C. for 30minutes at 45 millimeters pressure produced an initial permeability of10,400. Thus, a reduction in the heating temperatures should beaccompanied by increased duration of heating, in order to approximatethe same result.

In every case, if the iron is initially heat treated at a temperatureabove 900 C. and rapidly cooled (i. e. cooled at a rate of 100 C. perminute or more) improved results are obtained by an additional heattreatment at about 880 C.

A factor influencing the results obtained is the mass and shape of thematerial which efiects their cooling rates and the times required forannealing. Results given herein relate to relatively thin specimens.

Although this invention has been described with reference to but somerepresentative materials treated as has been set forth, it is understoodthat heat treatments in accordance with the invention may be employedfor improving the magnetic properties of many magnetic materials; infact, of the variety of materials treated, only iron incompletelydeoxidized by aluminum or containing aluminum, did not exhibit a largeimprovement in initial permeability. The fact that iron deoxidized byaluminum has not responded to the treatment is a fact of which the fullsignificance is not yet known. Aluminum as an impurity is sometimespresent in iron, but in accordance with the present invention it appearsto be an impurity to be avoided. However, if the iron is melted inhydrogen and subsequently the aluminum is added to the melt, a heattreatment of hard drawn wire made from the material at 1400 C. for 3hours in hydrogen at 760 m. in. pressure followed by rapid cooling,reannealing at 880 C. for one-half hour followed by slow coolingresulted in a #0 of 3,700 and ptmax. of 22,000. Therefore, it appearsthat the aluminum may be added after treatment of the molten iron inhydrogen with results not unsatisfactory.

In a particular instance, however, aluminum and silicon were presenttogether in a specimen treated with satisfactory results. Possibly thesilicon .obviated the detrimental effects of the aluminum.

Among the possibilities of application of materials treated inaccordance with this invention I may mention by way of example cores forrelays, transformers, particularly audio frequency transformers withimproved characteristics, magnetic clutches, power apparatus, receiverdiaphragms, etc. Choke coils of high inductance and low resistance maybe constructed in condensed volume with consequent saving of materialand cost of manufacture. The possibility of developing desired magneticproperties in magnetic elements when combined with other elements orparts which limit the temperature to which the material may be heated isa feature of the present invention.

Ordinary iron or other magnetic materials treated in accordance with theinvention may find a field of application as inductive loading materialfor signaling conductors, owing to the fact that their useful magneticproperties may be restored after the originally heat treated materialhas been subjected to detrimental influences simply by giving it asingle heat treatment at about 880 C. Either after or before havingreceived the first heat treatment at 1500 C. in hydrogen the magneticmaterial is formed by known methods into tape or wire of suitabledimensions. Then it is applied helically to a conductor as shown in Fig.12. The copper conductor I2 is surrounded by a plurality of conductivecopper strands l3 shaped to fit around the central conductor upon whichis wound the tape of loading material I4. Deleterious strains introducedinto the tapes or wires of the magnetic loading material during themachining and/or winding operations are relieved by giving the loadedconductor the second heat treatment at about 880 C. in hydrogen or not,as preferred. The invention therefore provides a method of continuouslyloading conductors with material of high initial permeability and otherdesirable magnetic properties.

So, also, for the production of finished machined parts in which it isdesirable or necessary to avoid the use of excessive temperatures, thehigh temperature hydrogen treatment may be given to the unprepared ironstock. Machining, drawing, or other operations such as are employed toproduce machined parts, transformer cores and relay parts may then beperformed, after which a simple annealing at about 880 C. will produce afinal product of excellent magnetic properties. In particular, the lowcoercivity of this material combined with high initial and maximumpermeability will enable certain classes of cores and relay parts to bemade with improved characteristics or cheapened or both.

Further illustrative of the results obtainable by the methods of thisinvention, a ring of Armco iron thick, 1 inside diameter, 1%" outsidediameter was heat treated in hydrogen at 1500" C. at 760 millimeterspressure for six hours, cooled to 880 C. in about 1 hour, thenreannealed in hydrogen at 880 C. for one hour and finally cooled slowlyin hydrogen in the furnace. It exhibited an initial permeability of 6000and a maximum permeability of 56,000.

In another case an Armco iron ring of {-3 inch inside diameter 1% inchoutside diameter and inch high was treated at 1480 C. for 18 hours,cooled to 880 C. in about one hour and heated at 880 C. in hydrogen for18 hours. It was found to have an initial permeability of 5000 and amaximum permeability of 280,000.

Materials treated as hereinbefore described may advantageously be usednot only where the new magnetic properties set forth are desired, butare also useful in cases where the new physical properties, such as thehigh degree of softness and/or the high metallurgical soundness, forinstance, are desirable, either per se or in combination with themagnetic properties.

A copending continuation in part hereof, Serial No. 173,956, filedNovember 11, 1937, has claims directed to the magnetic materialscomprising alloys of iron and molybdenum which are described, but notspecifically claimed, herein.

What is claimed is:

1. Method of preparing a magnetic material which comprises the step ofmelting the material in a hydrogenous atmosphere, followed by cooling toroom temperature, cold working the material, reannealing at atemperature of about 900 C. and cooling to room temperature.

2. Method of preparing magnetic iron which comprises the steps of firstmelting the iron in a hydrogenous atmosphere and then deoxidizing theiron by adding aluminum.

3. The method of improving the desirable properties of magneticmaterials, which comprises melting the materials in an atmospherecontaining hydrogen and reducing the partial pressure of the hydrogen toa point at which there occurs substantially no liberation of absorbedhydrogen during the solidification of said materials.

.4. Method as defined in claim 3 in which the partial pressure of thehydrogen gas is reduced to the value mentioned by substituting anatmosphere substantially composed of helium for the hydrogenousatmosphere.

5. The method of treating ordinary iron which comprises maintaining itin a hydrogenous atmosphere at a temperature between 1400 C. and themelting point and then cooling it to room temperature.

6. The method of improving the magnetic properties of magnetic materialswhich comprises maintaining them above 1400 C. in an atmosphereessentially of hydrogen for a period of time, working the material, andmaintaining the material at a temperature of about 880 C. for a periodof time in an atmosphere of hydrogen.

7. The method of producing a magnetic material consisting essentially ofunalloyed iron of improved magnetic properties which comprisesmaintaining the iron in a hydrogenous atmosphere at a temperaturebetween 1400 C. and the melting point at pressures of hydrogen gasranging from a small fraction of an atmosphere to above 12 atmospherescooling the iron, followed by annealing in an atmosphere of hydrogen ata temperature of about'880 C.

8. Iron which has been maintained in an atmosphere of hydrogen first ata temperature of between 1400 C. and the melting point for a period oftime and later at about 880 C. for a further period of time.

9. Commercial iron which has been heat treated in an atmosphere ofhydrogen above 1400 C. for a period of time and later annealed at about880 C. and which has a maximum permeability greater than 100,000.

10. Poly-crystalline iron which has been heat treated in an atmosphereof hydrogen at about 1500 C., and then later annealed at about 880 C.and has an initial permeability of at least 2500.

11. The method of improving the magnetic properties of magneticmaterials including iron and its alloys including its alloys with nickeland with silicon which comprises first heat treating the material in anatmosphere of hydrogen above 1400" C. and then heat treating thematerial at a temperature of about 880 C.

12. An article of manufacture comprising an industrial magnetic productcomposed of an ironnickel alloy which has been super-annealed above 1400C. in a hydrogen atmosphere, and again heat treated in a. hydrogenatmosphere'at about 880 C.

13. An iron-silicon alloy having up to about 4% silicon and theremainder essentially iron which has been maintained in a hydrogenatmosphere above 1400 C. for a period of time and later has beenannealed at a temperature of about 1000 C. and has an initialpermeability of at least 4000.

PAUL P. CIOFFl.

